Tagatose is a natural sweetener that is present in small amounts in foods such as milk, cheese, cacao, etc., and sweet and natural fruits such as apples and tangerines, and also has physical properties similar to those of sugar. Tagatose has a calorie of 1.5 kcal/g, which is about one-third of sugar, and has a glycemic index (GI) of 3, which is about 5% of the sugar. Tagatose has a variety of health functionalities with sweetness similar to sugar, and thus, tagatose may be used as a substitute sweetener capable of satisfying health and taste at the same time when applied to various products.
Conventionally known methods for production of tagatose include a chemical (catalytic reaction) method and a biological (isomerization enzyme reaction) method using galactose as a main raw material (see Korean Patent Laid-Open Publication No. 2009-0082774). However, lactose, which is a basic raw material of galactose in the above-described production method, is unstable in price due to a production amount, demand and a supply amount of raw milk and lactose in the international market, and thus there is a limit to stably meet supply and demand of raw materials for production of tagatose. Therefore, a new method in which tagatose is able to be produced by using common generic sugars (sugar, glucose, fructose, etc.) as raw materials has been demanded. Accordingly, the present inventors have reported a production method for tagatose from fructose using a novel hexuronate C4-epimerase (Korean Patent Laid-Open Publication No. 10-2014-0143109). However, for industrial production, it is necessary to develop an enzyme having higher conversion activity to tagatose.
Under these circumstances, the present inventors confirmed that when an amino acid at a specific position of the hexuronate C4-epimerase was mutated, the conversion activity from fructose to tagatose was remarkably increased as compared to that of the wild-type, and completed the present invention.
Throughout the present specification, a number of patents and documents are referenced and the citation is shown in parentheses. The disclosures of these patents and publications are hereby incorporated by reference in their entirety to more clearly illustrate the present invention and the level of the technical field in which the invention pertains.